Serum transferrin is a 80 kD protein that binds two iron atoms for transport through the blood and, after binding to the transferrin receptor, endocytosis into proliferating cells. Iron release from serum transferrin is dependent on many parameters, in particular, on the pH drop in the endosomes from physiological pH to 5.5. We propose to investigate the global structural changes occuring during iron release by using time-resolved SAXS. Specifically, we will measure the time course change in Rg and Rc of the protein after initiating a pH drop and correlate the changes in these structural parameters with our data from time-resolved spectroscopic studies.